1. Field of the Invention
The present invention relates to a method for measuring and controlling oscillations in a wind turbine coupled directly to a power mains, voltage and/or current being measured at the mains connection and other values being optionally derived therefrom, such as apparent power, real power, reactive power, power factor, and cos(xcfx86).
2. Description of the Related Art
The development of wind turbines leads to increasingly bigger turbines with increasing hub heights and rotor diameters. This creates a number of mechanical difficulties caused by the correspondingly lower natural resonance frequencies and attenuations of the structures used.
Furthermore, certain rotor blades in common use show a tendency to form a complex pattern of rotor blade oscillations of great amplitude under certain meteorological conditions. This exposes the rotor blades to unacceptable dynamic loads reducing their time of life and endangering the wind turbine safety.
It is essential for the structural safety of the wind turbine and thus also for human safety to be able to detect both oscillations of the wind turbine rotor blades and other structural oscillations of the wind turbine and to be able to stop these oscillations, e.g. by a temporary wind turbine standstill or in any other way.
A general wind turbine standstill under meteorological conditions that appear to facilitate the oscillation phenomena is not a practicable solution as it would be detrimental to the annual output of the wind turbine. It is therefore desirable to be capable of detecting the presence or absence of the oscillation phenomena and to be capable of adjusting the wind turbine control means so as to stop any occurring oscillation phenomenon so that the wind turbine may resume normal operation once the oscillation phenomenon has ceased.
Certainly all wind turbines have been provided with a device capable of detecting heavy oscillations and bringing the turbine to a standstill by interacting with the wind turbine emergency stop circuit. However, this device is primarily designed to ensure stopping the wind turbine in case of a sudden rotor unbalance such as by loss of a blade tip or in case of structural damage to the rotor following a lightning stroke. This prior art device cannot detect rotor blade oscillations or tower oscillations until they have reached an amplitude which presents an immediate danger to the wind turbine. Further, in the case of rotor blade oscillations even if the oscillation phenomenon of a single blade reaches a harmful amplitude, the resulting vibration level as measured e.g. at the wind turbine nacelle is relatively modest as the phenomenon generally comprises oscillations in opposite phase on two out of three rotor blades. Therefore such a device is not suitable for detecting such a harmful situation.
Further, this prior art device is generally incapable of making the wind turbine resume normal operation once the oscillations have ceased, since the device is incorporated as a part of the emergency stop circuit of the wind turbine.
Finally, self-contained vibration guards are known that may be incorporated in wind turbines. These guards permit detecting the presence of e.g. oscillations at a modest amplitude so that the dynamic load on the rotor blades can be kept within acceptable limits provided that the turbine can be stopped at the command of the vibration guard.
However, these self-contained units are normally also incorporated in the emergency stop circuit of the wind turbine and thus also cannot bring the wind turbine into normal operation once the oscillations have ceased.
For the purpose of wind turbine approval, it is also desirable that detection and countermeasures against oscillation phenomena are effected at a modest oscillation amplitude by the wind turbine control system itself not depending upon external devices.
It is an object of the present invention to provide a method for detecting on the basis of existing measurement signals the existence of wind turbine oscillations and for attenuating these oscillations by means of the wind turbine control system.
This object is achieved by a method of the type mentioned initially, in which a spectrum analysis is performed of one or more of the measured values or any values derived therefrom, and wind turbine oscillation amplitudes are determined on the basis of the result of the spectrum analysis, and when the determined amplitude exceeds a certain value, countermeasures are taken by the wind turbine control system to reduce the amplitude of the oscillations.
This permits detection of even minor vibrations or oscillations without incorporation of external devices in the wind turbine by exploitation of the fact that the vibrations or oscillations lead to variations in the electrical measurements already being made on the wind turbine or values derived therefrom, and permits using the result of the detection to attenuate the detected vibrations or oscillations by means of the wind turbine control system.
The dependent claims define appropriate embodiments of the method according to the invention. Particularly the dependent claims 4-10 define appropriate methods of counteracting for reducing the amplitude of the detected oscillations relating to the particular type of control system provided in a wind turbine.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.